1. Field of the Invention
The present invention relates to an exhaust pipe assembly for a multi-cylinder internal combustion engine, in which exhaust pipes to be connected to a head of the engine are made of steel pipes.
2. Description of the Related Art
The structure of an exhaust pipe assembly in which steel pipes are used in making exhaust pipes is shown in, e.g., Japanese Published Unexamined Utility Model Registration Application No. 1819/1993, as illustrated in FIGS. 6A through 6C. In FIG. 6A, reference alphabet "a" denotes a flange which is connected to a cylinder head of an internal combustion engine, reference alphabets b denote exhaust pipes which are made of steel pipes, alphabet c denotes a collecting case (or an assembly casing), and alphabet d denotes a flange which is connected to an exhaust gas purifier. Downstream ends of the exhaust pipes b are shaped into sectors (or fan shapes) b.sub.1 as shown in FIG. 6B. The V-shaped portions (i.e., the portions formed by two radii) of the respective fan-shaped portions b.sub.1 are welded together and are converged such that a series of arched portions substantially form a circle. The subassembly thus obtained is fitted into the collecting case c, and the subassembly and the collecting case are welded together. In FIG. 6C, a welded portion W.sub.1 shows a cross-shaped welding performed along the V-shaped portions. A welded potion W.sub.2 shows a welding performed between the arched portions and the collecting case.
In these welded portions, the cross-shaped welded portion W.sub.1 must be welded along the entire length of the V-shaped portions so that the exhaust gases do not leak under pressure. In the welded portion W.sub.2, attention must be paid so that gases do not leak under pressure through clearances e which are formed on outer corners of the arched portions. This kind of welding work is not easy to perform and is low in workability (or welding efficiency). In addition, there is a disadvantage in that welding defects, if any, cannot be easily located and repaired. Furthermore, in this example, since the above-described V-shaped portions are assembled on a back-to-back relationship in as many as at four portions, heat is accumulated to thereby raise the temperature therein. As a result, the temperature difference between the V-shaped portions and the outer arched portions becomes so large as to cause cracks. In order to minimize the possibility of occurrence of cracking, a high-quality and heat-resistant material must be used in the exhaust pipes b.
FIGS. 7A and 7B show another example of the conventional exhaust pipe assembly, in which an intermediate flange f is interposed between the exhaust pipes b and the collecting case c. The intermediate flange f is made by forming holes f.sub.1 for receiving therein respective exhaust pipes b. The respective exhaust pipes b are then inserted into the holes f.sub.1 and are welded to the flange f at welding portions W.sub.3. This intermediate flange f is thereafter welded to the collecting case c at a welding portion W.sub.4. In this example, the intermediate flange f must be provided, as shown in FIG. 7B, with an allowance (or a required length) t.sub.1 for inserting the pipe as well as an allowance t.sub.2 for welding. Therefore, the thickness T of the intermediate flange f becomes large and the heat is accumulated therein, with the result that the intermediate flange f is likely to be subject to a damage by the heat. To minimize this kind of disadvantage, a thicker and highly heat-resistant material must be used, resulting in a lager weight. In addition, if the distance 1 between the holes f.sub.1, f.sub.1 for inserting the exhaust pipes b is made smaller in an attempt to minimize the weight of the intermediate flange f, the strength of a metallic mold for use in punching the intermediate flange f becomes so low that the punching of the holes can no longer be performed. As a solution to this problem, the holes f.sub.1, f.sub.1 for inserting the pipes b must be made by an expensive way of working in the form of cutting. Even if these holes are formed by cutting work, the intermediate flange f is likely to cause cracks at welding beads due to thermal strains by welding, if the distance 1 is minimized.
The present invention has an object of providing an exhaust pipe assembly for a multi-cylinder internal combustion engine in which the members for assembling the plurality of exhaust pipes together can be made in a material which is less likely to accumulate heat and which is small in weight and less expensive.